The fate of technetium in reduced estuarine sediments: Combining direct and indirect analyses

Technetium-99 is an important fission product in radioactive wastes. As Tc ( VII ) O 4 - , Tc is highly mobile in oxic environments but, under reducing conditions, Tc becomes strongly associated with sediments as hydrous Tc(IV)O2 like phases. In order to further examine the behaviour of Tc over a range of concentrations in estuarine sediments, anoxic incubation experiments were combined with a range of direct (transmission electron microscopy and gamma camera imaging) and indirect (incubation experiments and chemical extractions) experimental techniques. When TcO4− was incubated in sediment microcosms at micro-molar (10−6 mol L−1) concentrations, >99% Tc O 4 - was removed from solution over the course of 36 days in systems undergoing active microbial Fe(III)-reduction. By contrast, when spiked into pre-reduced estuarine sediments that were predominantly Fe(III)-reducing (incubated for 60 days) or SO 4 2 − -reducing (incubated for 270 days), >99% Tc O 4 - was removed from solution in under 10 min in both microbially active and heat sterilised systems. Chemical extraction techniques showed that 70 ± 3% of Tc bound to sediments was remobilised when sediments were exposed to the first strong oxidant (H2O2) in the extraction scheme. At higher Tc concentrations (∼0.05 mol kg−1 of sediment) scanning transmission electron microscopy, combined with energy dispersive X-ray mapping, was used to examine the associations of Tc in sediments. At these concentrations, Tc was localised and co-associated with nanometre size Fe(II)-rich particles, consistent with the hypothesis that removal of Tc may be controlled by reduction of Tc(VII) to Tc(IV) by biogenic Fe(II) in sediments. In addition, gamma camera imaging with the γ-emitting 99 m Tc O 4 - (half-life 6 h) at pico-molar (10−12 mol L−1) concentrations, was used to visualise the interaction of Tc in sediments at very low concentrations. Here, over the course of 24 h the scavenging of Tc to SO 4 2 − -reducing sediments was observed. As the Tc concentrations used in the 99mTc experiments were below the solubility limits for hydrous Tc(IV)O2 (ca. 10−9 mol L−1 at pH 7–9), sorption of Tc(IV) species is likely to be a significant control on Tc behaviour in these sediments even at very low concentrations. Overall, the results of this study show that multiple approaches are essential to understanding Tc speciation in complex heterogeneous sediments over the wide range of concentrations relevant to contaminated natural and engineered environments.